This invention relates to a push-pull amplification circuit, particularly to a bias stabilizing technique for an output circuit section thereof.
An output stage of general power amplifiers of push-pull type has a configuration as shown in FIG. 9. An npn transistor Q11 and a pnp transistor Q12 are output transistors, and a bias circuit consisting of resistances RB1, RB2 and an npn transistor Q13 is provided in order to supply them with a predetermined bias. A base-base voltage VBB of the output stage transistors Q11 and Q12 is determined by setting a ratio of the resistances RB1 and RB2, whereby an idling current is determined.
In such a prior art power amplifier output stage configuration, it is necessary to provide a heat coupling of the biasing transistor Q13 with the output stage transistors Q11 and Q12 in order to prevent a heat trouble caused by a heat generation of the transistors Q11 and Q12, that decreases base-emitter voltage (VBE) of the transistors resulting in the current increase. Emitter resistance RE of the output stage transistors Q11 and Q12 have a negative feedback function against the increase in current of these transistors. Since, however, in general power amplifiers, each of emitter resistance RE has an extremely small value of 1 ohm or below, the current can be remarkably affected by a small change in the bias voltage and base-emitter voltage VBE. Therefore, in general, the heat coupling as described above is regarded to be indispensable.
As the idling current can also be largely affected by irregularity in element characteristics, the bias resistances RB1 and RB2 should not be fixed, but be adjustable. For example, in case that output transistors are replaced, readjustment of the bias resistances RB1 and RB2 is required.
Moreover, in order to provide the heat coupling of the bias transistor Q13 with the output transistors Q11 and Q12, it is necessary to locate the bias transistor Q13 on heat radiators for the output transistors Q11 and Q12, for which a special structural arrangement are required.
In order to solve these problems as described above, a technique as shown, for example, in the FIGS. 3 and 4 of the Japanese Patent Publication No. Sho 54-12031 is proposed, in which the base bias of the output transistors is control led by detecting voltage across emitter resistance RE of the output transistors.
However, the resistances RE inserted between the emitters of the output transistors of push-pull type which are originally designed for achieving an operation balance have a very small value of 1 ohm or less. Accordingly, in case of a small current operation, different from a case of large amplitude operation, change in a detected voltage by the emitter resistance is extremely small and, therefore, it is difficult to secure accuracy in bias control. Further, it is problematical to increase the emitter resistance RE in order to increase the detected voltage, because output impedance of the amplifier is increased.
As described above, there are problems in the conventional output stage configuration of general power amplifiers, that adjustment of idling is required, temperature compensation by the heat coupling is indispensable, and that some structural arrangement for them is needed. In the method in which the bias control is made by the voltage across emitter resistance of the output transistors, it is difficult to perform an accurate control, and if the emitter resistance is to be increased to realize an accurate control, it results in deterioration of the amplifier characteristics such as increase in the output impedance.
This invention aims at offering an push-pull amplification circuit, which does not cause deterioration of characteristic or require the idling adjustment or the temperature compensation.